As a drive circuit for driving on/off an insulated-gate semiconductor element such as an IGBT, for example, as shown in FIG. 5, there is known a drive circuit 1 for an insulated-gate semiconductor element which controls an output current Iout to be supplied to the gate of the IGBT in accordance with a drive signal. The drive circuit 1 schematically includes a current source 3 for generating the output current Iout to be supplied to the gate of the IGBT 2, and a current output circuit 4 for controlling, in accordance with the drive signal, the supply of the output current Iout generated by the current source 3.
Incidentally, the drive circuit 1 includes an output current control circuit 5 for defining a magnitude of the output current Iout outputted from the current source 3. The output current control circuit 5 includes, for example, an operational amplifier OP1 for controlling a current Io flowing through an N-type MOS-FET 5a, and a reference resistor (Rref) 5b for generating a current detection voltage according to the current Io flowing through the MOS-FET 5a. The operational amplifier OP1 plays a role of controlling the gate voltage of the MOS-FET 5a in accordance with a voltage difference between the current detection voltage generated between both terminals of the reference resistor 5b and a predetermined reference voltage Vref, thereby making the current Io flowing through the MOS-FET 5a constant.
The current source 3 is realized as a P-type MOS-FET 3a which constitutes a current mirror circuit in combination with a P-type MOS-FET 5c, which is connected as a load of the MOS-FET 5a to the drain of the MOS-FET 5a, and generates the output current Iout proportional to the current Io.
The current output circuit 4 includes an N-type MOS-FET 4b which is interposed between the gate of the IGBT 2 and the ground (GND) and turned on/off by a buffer 4a which receives the drive signal as input. Further, the current output circuit 4 includes a P-type MOS-FET 4c which is connected in parallel to the MOS-FET 3a constituting the current source 3 and a level shift circuit 4d which turns the MOS-FET 4c on/off in accordance with the drive signal.
When the drive signal is at a high (H) level, the current output circuit 4 turns the MOS-FET 4c on via the level shift circuit 4d, thereby stopping a function of the current mirror circuit and thus stopping the current output from the current source 3, and also turns the MOS-FET 4b on via the buffer 4a. Then, the current output circuit plays a role of discharging an electric charge stored in the gate of the IGBT 2 via the MOS-FET 4b, thereby turning the IGBT 2 off.
When the drive signal is at a low (L) level, the current output circuit 4 turns the MOS-FET 4c off via the level shift circuit 4d, thereby outputting the output current Iout from the current source 3, and also turns the MOS-FET 4b off via the buffer 4a. As a result, the output current Iout from the current source 3 is supplied to the gate of the IGBT 2 to turn the IGBT 2 on. In this manner, the current output circuit 4 controls the turning-on/off of the IGBT 2 in accordance with the drive signal.
The switching characteristics of the IGBT 2 which is driven on/off by the drive circuit 1 change depending on an operating temperature (temperature T) of the IGBT 2. In particular, a switching loss at the time of turning-on of the IGBT 2 is likely affected by the output current Iout supplied to the gate of the IGBT 2 and a change in the operating temperature.
In the conventional drive circuit 1, for example, as disclosed in Patent Document 1, it is proposed that information of the operating temperature of the IGBT 2 is fed back to the current output circuit 4, and the output current Iout generated by the current source 3 is subjected to a feedback control according to the operating temperature of the IGBT 2. By such a feedback control of the output current Iout according to the operating temperature of the IGBT 2, it is possible to achieve the reduction of the switching loss of the IGBT 2 and the reduction of switching noise thereof.
In the case of evaluating the output current characteristics of the drive circuit 1 configured in this manner, a current detection circuit (test device) solely capable of measuring the output current Iout of the drive circuit 1 is required to be used. This kind of current detection circuit (test device) is disclosed in detail in, for example, Patent Document 2, and the like. The current detection circuit (test device) disclosed in Patent Document 2 is configured to sample an output current Iout of a measurement object (drive circuit 1 in this case) and evaluate the magnitude the output current, as shown in FIG. 3.